1. Field of the Invention
The present invention relates to compositions having improved wear resistance characteristics.
2. Background Art
In drilling oil and gas wells or mineral mines, earth-boring drill bits are commonly used. Typically, an earth-boring drill bit is mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface. With weight applied to the drill string, the rotating drill bit engages an earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A rock bit, typically used in drilling oil and gas wells, generally includes one or more rotatable cones (also referred as to “roller cones”) that perform their cutting function through the rolling and sliding movement of the cones acting against the formation. The cones roll and slide upon the bottom of the borehole as the bit is rotated, thereby engaging and disintegrating the formation material in its path. A borehole is formed as the gouging and scraping or crushing and chipping action of the rolling cones removes chips of formation material that are then carried upward and out of the borehole by circulation of a liquid drilling fluid or air through the borehole. Petroleum bits typically use a liquid drilling fluid which is pumped downwardly through the drill pipe and out of the bit. As the drilling fluid flows up out of the borehole, the chips and cuttings are carried along in a slurry. Mining bits typically do not employ a liquid drilling fluid; rather, air is used to remove chips and cuttings.
The earth-disintegrating action of the roller cone cutters is enhanced by a plurality of cutter elements. Cutter elements are generally inserts formed of a very hard material and are press-fit into undersized apertures or sockets in the cone surface. Due to their toughness and high wear resistance, inserts formed of tungsten carbide dispersed in a cobalt binder have been used successfully in rock-drilling and earth-cutting applications.
Breakage or wear of the tungsten carbide inserts limits the lifetime of a drill bit. The tungsten carbide inserts of a rock bit are subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impacting loads from contact with the borehole bottom. Also, the high wear load can cause thermal fatigue in the tungsten carbide inserts which can initiate surface cracks on the inserts. These cracks are further propagated by a mechanical fatigue mechanism caused by the cyclical bending stresses and/or impact loads applied to the inserts. This may result in chipping, breakage, and/or failure of inserts.
Inserts that cut the corner of a borehole bottom are subject to the greatest amount of thermal fatigue. Thermal fatigue is caused by heat generation on the insert from a heavy frictional loading produced as the insert engages the borehole wall and slides into the bottom-most crushing position. When the insert retracts from the borehole wall and the bottom of the borehole, it is quickly cooled by the circulating drilling fluid. This repetitive heating and cooling cycle can initiate cracking on the outer surface of the insert. These cracks are then propagated through the body of the insert when the crest of the insert contacts the borehole bottom, as high stresses are developed. The time required to progress from heat checking to chipping, and eventually, to breaking inserts depends upon formation type, rotation speed, and applied weight.
Thermal fatigue is more severe in mining bits because more weight is applied to the bit and the formation usually is harder, although the drilling speed is lower and air is used to remove cuttings and chips. In the case of petroleum bits, thermal fatigue also is of serious concern because the drilling speed is faster and liquid drilling fluids typically are used.
Cemented tungsten carbide (also known as sintered tungsten carbide) refers to a material formed by mixing particles of tungsten carbide, typically monotungsten carbide, and particles of cobalt or other iron group metal, and sintering the mixture. In a typical process for making cemented tungsten carbide, small tungsten carbide particles, e.g., 1–15 microns, and cobalt particles are vigorously mixed with a small amount of organic wax which serves as a temporary binder. An organic solvent may be used to promote uniform mixing. The mixture may be prepared for sintering by either of two techniques: it may be pressed into solid bodies, often referred to as green compacts; alternatively, it may be formed into granules or pellets, such as by pressing through a screen or tumbling, and then screened to obtain more or less uniform pellet sizes.
Such green compacts or pellets are then heated in a vacuum furnace to first evaporate the wax and then to a temperature near the melting point of cobalt (or the like) to cause the tungsten carbide particles to be bonded together by the metallic phase. After sintering, the compacts are crushed and screened for the desired particle size. Similarly, the sintered pellets, which tend to bond together during sintering, are crushed to break them apart. These are also screened to obtain a desired particle size. The crushed cemented carbide is generally more angular than the pellets, which tend to be rounded.
Another type of tungsten carbide is macro-crystalline carbide. This material is essentially stoichiometric WC. Most of the macro-crystalline tungsten carbide is in the form of single crystals, but some bicrystals of WC may also form in larger particles. Single crystal monotungsten carbide is commercially available from Kennametal, Inc., Fallon, Nev.
Carburized carbide is yet another type of tungsten carbide. Carburized tungsten carbide is a product of the solid-state diffusion of carbon into tungsten metal at high temperatures in a protective atmosphere. Sometimes it is referred to as fully carburized tungsten carbide. Such carburized tungsten carbide grains usually are multi-crystalline, i.e., they are composed of WC agglomerates. The agglomerates form grains that are larger than the individual WC crystals. These large grains make it possible for a metal infiltrant or a infiltration binder to infiltrate a powder of such large grains. On the other hand, fine grain powders, e.g., grains less than 5 μm, do not infiltrate satisfactorily. Typical carburized tungsten carbide contains a minimum of 99.8% by weight of WC, with a total carbon content in the range of about 6.08% to about 6.18% by weight.
In addition, U.S. Pat. No. 5,880,392, which is assigned to the assignee of the present application and is hereby incorporated by reference, discloses another type of carbide, which are known as double cemented carbides. Double cemented carbide composites disclosed in that patent comprise a plurality of first regions and a second ductile phase that separate the first regions from each other. Each first region comprises a composite of grains and a first ductile phase bonding the grains. The grains are selected from the group of carbides consisting of W, Ti, Mo, Nb, V, Hf, Ta, and Cr carbides. The first ductile phase is selected from the group consisting of Co, Ni, Fe, alloys thereof, and alloys with materials selected from the group consisting of C, B, Cr, Si and Mn.
A preferred first region comprises tungsten carbide grains that are cemented with a cobalt first ductile phase. The second ductile phase is selected from the group consisting of Co, Ni, Fe, W, Mo, Ti, Ta, V, Nb, alloys thereof, and alloys with materials selected from the group consisting of C, B, Cr, and Mn. A preferred second ductile phase is cobalt. Additionally, additives such as those selected from the group consisting of carbides, nitrides, and borides can be added to the second ductile phase to provide improved properties of wear resistance.
U.S. Pat. No. 5,880,382 discloses preparing double cemented carbides by combining hard phase particles (e.g., WC—Co), formed from the grains and first ductile phase, with the second ductile phase material under conditions of pressure and heat. The composite comprises in the range of from about 40 to 95 percent by volume first phase (the hard phase particles), and less than about 60 percent by volume second ductile phase based on the total volume of the composite, and more preferably the composite comprises in the range of from about 60 to 80 percent by volume first phase and in the range of from about 20 to 40 percent by volume second ductile phase based on the total volume of the composite. Composite embodiments comprising an additive in the second ductile phase comprise less than about 40 percent by volume of the additive based on the total volume of the second ductile phase.
Double cemented carbide composites have improved properties of fracture toughness when compared to conventional cemented tungsten carbide materials, without sacrificing wear resistance, i.e., having equal or better wear resistance than that of conventional cemented tungsten carbide materials, making the material well suited for such applications as roller cone bits, percussion or hammer bits, drag bits, and other applications such as mining and construction tools where properties of improved fracture toughness is desired.
However, in certain applications, double cemented carbides have been found to lack sufficient wear resistance. What is needed, therefore, are methods and compositions that increase the wear resistance, while maintaining fracture toughness.